This invention relates generally to aircraft navigation, and, more particularly, to following a flight plan from an offset course position.
An onboard computer on an aircraft contains guidance and flight director algorithms that permit the pilot to fly a flight plan by maneuvering the aircraft so that the flight director cues shown on the multi-function display (MFD) are nulled or centered. This approach permits the pilot to accurately follow an alternate flight plan parallel to the original flight plan with reduced workload. An autopilot, when the aircraft is so equipped, can also be used to accurately follow the alternate flight plan in place of the flight director. Hence, the offset course design to be described below is applicable to lateral control of an aircraft using either a flight director or an autopilot. The Army, however, discovered shortcomings in the current special operation aircraft (SOA) design for certain mission operations.
A first problem is presented when using the SOA system to fly multiple aircraft in formation where all the aircraft have the same flight plan stored in the onboard computers. A formation consists of a leader aircraft and one or more follower aircraft. Over the course of a mission, different aircraft from the formation may assume the role of the leader. Follower aircraft are commonly behind and offset laterally from the leader. For the current SOA design, the guidance function of the follower aircraft treats the desired lateral offset as a cross track error. Hence, the pilot of the follower aircraft cannot use the flight director or autopilot to fly an offset position because centering the lateral cue on the MFD forces the aircraft back onto the original flight plan leg.
Another problem in using the current SOA system is the lack of a simple procedure to quickly alter a portion of the flight plan while in flight as a means to get around large obstacles such as an unanticipated threat or a storm.
In the SOA guidance system, the flight plan is stored as a table in the mission management function. This table contains the waypoint coordinates (latitude and longitude) plus the desired ground speed and altitude for each leg of the flight plan. Lateral guidance compares the actual aircraft position and ground track angle estimated by the navigation function to the desired ground track. The resulting lateral guidance errors are the cross track distance, cross track velocity, and track angle error. In a similar manner, longitudinal and vertical guidance compute the speed error and altitude error, respectively, by comparing the aircraft speed and altitude to the desired speed and altitude for the current leg.
The guidance errors are inputs to the flight director control rules which in turn compute the steering cues displayed on the MFD. The pilot closes the flight control feedback loop by adjusting the cockpit flight controls (i.e., cyclic and collective inputs) to center the steering cues. When the cues are centered, the aircraft accurately follows the desired ground track, leg speed, and leg altitude. Digital avionics also provide the flight crew with a Horizontal Situation Display (HSD) showing a top down view of aircraft position relative to the flight plan ground track and waypoints.
An important aspect of the lateral guidance design is the logic controlling the turn from one flight plan leg to the next. The onboard computer automatically initiates turns onto the next leg at the proper time, and two types of turns are permitted. The particular turn choice for each destination waypoint is stored in the mission management function as part of the flight plan.
One choice of turn is a xe2x80x9cflyover turnxe2x80x9d where the switch to the next leg occurs when the aircraft xe2x80x9ccapturesxe2x80x9d the destination waypoint. Hence, the aircraft does not start turning until crossing the next leg. This means the flyover turn results in the aircraft overshooting the ground track of the next leg before getting back on course.
The other turn type is called xe2x80x9clead turnxe2x80x9d where the aircraft smoothly rounds the corner formed by the adjacent flight plan legs. This is accomplished by starting the turn at a point prior to the destination waypoint where the turn point offset distance is a function of turn angle and the aircraft turn radius. Upon capturing this offset turn point, mission management switches to the next leg. The resulting flight director steering cues cause the pilot to roll the aircraft so that it rounds the corner with no overshoot of the next leg.
A first approach considered in solving the offset course guidance problems is based on constructing a second flight plan corresponding to the offset course. The transition from the original to offset course and vice versa was accomplished by switching between the two flight plans. This straightforward approach requires that the latitude and longitude of the offset waypoints be computed from the offset distance and the latitude and longitude of the original waypoints. The desired leg ground speeds would have to be recomputed because the leg lengths for the original and offset course are usually different. A first disadvantage of this method is the additional computer time and storage required to use explicit offset waypoints. A second disadvantage of this approach is the extensive effort required to modify the complex mission management software and validate the new design.
It remains desirable to have a system and method for following an offset course in an aircraft without having to explicitly modify or recompute the original flight plan.
It is an object of the present invention to provide a method and system that enables a pilot to fly an offset course as easily as flying an original course.
It is another object of the present invention to provide a method and system to make turning from a first flight plan leg in an offset course to a second flight plan leg in the offset course automatic and efficient.
The problems of establishing and following an offset course for aircraft are solved by the present invention of an offset course guidance system using a shadow aircraft.
In the present invention, an imaginary aircraft, called the xe2x80x9cshadow aircraft,xe2x80x9d flies the original flight plan and, in turn, causes the true aircraft to fly the offset course.
The offset course has the same number of legs as the original course, and for each leg of the original course, there is a corresponding parallel leg of the offset course. Except for an initial waypoint and a final waypoint, the locations of offset course waypoints are defined to be the intersections of the straight lines parallel to the original legs at the specified offset distance. The range and bearing to the offset course initial waypoint are chosen to be the same as the range and bearing to the next waypoint. For the interior legs of the flight plan, the lengths of the offset course legs vary from the corresponding xe2x80x9ctruexe2x80x9d course leg. A maximum offset distance, a minimum course leg length, and an allowable region of transition are defined in the offset course system in order to be able to acquire and track a xe2x80x9cflyablexe2x80x9d offset course. The resulting system enables an aircraft to fly an offset course that is offset laterally from the true aircraft position without computing and storing offset course geometry in the form of latitude and longitude of the offset waypoints.
The present invention together with the above and other advantages may best be understood from the following detailed description of the embodiments of the invention illustrated in the drawings, wherein: